This invention relates to the fabrication of microporous membranes, especially to the fabrication of microporous polybenzimidazole membranes having a relatively narrow pore size distribution and uniform porosity.
Porous membranes are well-known to those skilled in the art. Generally they comprise thin films or hollow fibers made from organic polymers. These membranes contain voids (pores) which are quite large in comparison to molecular dimensions. The pore size may vary over a wide range and the porosity may not be uniform from one membrane area to another. The essential transport characteristics of porous membranes depend upon the pore structure. Selectivity depends on the size of the molecule relative to the pores, as well as on mutual attractions and repulsions between the membrane and the molecules or particles passing therethrough.
Microporous membranes typically have pores between about 0.005 and 20 microns in diameter. Their uses include filtration and separation applications. These membranes can be used for the separation of very small particles, such as colloids and polymers, either from each other or from the liquid in which they are suspended. They are also useful as separators in rechargable batteries, wherein two electrodes must be physically separated from each other in such a way that there is a continuous pathway for exchange of small ions without the mixing of reactants and products of the two half-cell reactions. These types of membranes can also be used in applications in which gas diffusion is desired, as in blood oxygenators, wherein the membrane has a liquid in contact with one surface and a gas in contact with the other surface. Typically, the efficiency and selectivity of these membranes are limited by a wide pore size range and by nonuniformities in the pore structure.
A process for the production of microporous polybenzimidazole articles is disclosed in U.S. Pat. No. 4,828,699. According to this process, membranes may be made by dissolving polybenzimidazole and a leachable additive in an appropriate solvent, depositing this solution onto a support to form a wet film and evaporating solvent to provide a thin solid layer. The membrane is made microporous by washing with a solvent that dissolves the additive and removes the remaining traces of the original solvent but does not dissolve PBI. This method is somewhat limited in that only relatively thin films may be produced; furthermore, it requires dissolving the PBI.